This paper gives you information such as distance sensor, wavelength, position feedback, sensors, interferometric sensors, and bandpass filter. The following are excerpted from this paper:

Development of an in-fiber white-light interferometric distance sensor for absolute measurement of arbitrary small distances Ayan Majumdar and Haiying Huang* Mechanical and Aerospace Engineering Department, University of Texas at Arlington, 500 W. Therefore, the LPFG-based distance sensor is similar to the FPI distance sensor except that its OPD is contributed to by both the LPFG cavity distance L and the FPI cavity distance, which is equal to the target distance d.

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